The present invention relates generally to cell equalization using state-of-charge (SOC) and, in particular, to cell equalization in a multi-cell battery pack.
Batteries are used in a wide variety of electronic and electrical devices. In the context of rechargeable battery pack technologies with multiple cells, it is often desirable to balance or equalize the cells in the battery pack. For example, it is desirable to balance or equalize cells that are configured in series strings. Cells are configured in series strings in order for battery packs to achieve high power levels with a reasonable current source/drain. The higher voltage of a series string allows a lower current for the same power level. Generally, in the prior art, the terms “balancing” and “equalizing” refer to the process of causing a terminal voltage of all the cells to converge to a constant level.
Over time, a battery pack's cells may become “out of balance” as small differences in their individual dynamics—principally, in their Coulombic efficiencies and capacities—cause their states-of-charge to drift apart from each other as the pack operates. Unfortunately, one or more cells may eventually limit the discharge ability of the pack by having state-of-charge (SOC) much lower than that of the others, and/or one or more cells may limit the charging capacity of the pack by having SOC much higher than that of the others. In an extreme case, the pack becomes incapable of either charge or discharge if one cell is at the low SOC limit and another is at the high SOC limit, even if all other cells have intermediate SOC values. Packs may be balanced or equalized by “boosting” (individually adding charge to) cells with SOC too low, “bucking” or “shunting” (individually depleting charge from) cells with SOC too high or “shuffling” (moving charge from one cell to another).
In some applications, for example, those with long discharge periods followed by a complete charge, equalization is only performed at the end-of-charge point in a charging process and continues until the pack is fully balanced. Other applications, however, require that the pack undergo continuous partial-charge and partial-discharge periods, and so equalization needs to be performed as the pack operates where cells in the battery pack have their charge levels adjusted continuously in a direction leading toward a fully balanced pack. Equalization may be halted if cells become close enough to full balance for the problem at hand, and may be resumed if the cells become significantly out of balance again.
In the prior art, determining which cells should have their charge levels adjusted to equalize the pack is generally done on the basis of cell voltage alone. If all cell voltages are the same, perhaps within some tolerance, the pack is considered properly balanced. If a cell's voltage is too high, then charge needs to be depleted from the cell. If a cell's voltage is too low, then charge needs to be added to the cell. Various electronic means are available to perform the equalization, either automatically, or under microprocessor control. These include: shuffling charge, depleting charge, and adding charge. For shuffling charge, charge is moved from one or more cells with voltage too high to one or more cells with voltage too low. A switched capacitor may be used or an energy-transfer method based on transformer windings may be used. For depleting charge, charge is depleted from one or more cells with voltage too high (e.g., with a switched resistor). For adding charge, charge is added from an external source, or from the pack itself (e.g., with a DC-DC converter). A disadvantage of using cell voltage as an indicator of when to perform equalization, as well as to control the cell equalization, is that it fails to maximize performance of the battery pack.
The purpose of equalization is to maintain the battery pack in a state where the maximum level of charge and discharge power is available for use. There is a need for a method of improving performance of a battery pack by boosting or bucking or shuffling charge among cells to maximize the pack availability.